The present invention relates to a fuel assembly, and particularly to a fuel assembly suitable for use in a boiling water reactor (referred to as BWR hereinafter) and a reactor core thereof.
A conventional BWR comprises a core which is constituted by a plurality of fuel assemblies arranged at equal intervals in the form of a lattice. A control rod having a cruciate cross-sectional form is inserted between the fuel assemblies. Such fuel assembly that is loaded in the conventional BWR core comprises fuel rods which are arranged in 8 rows and 8 columns (8.times.8) in a channel box and two water rods which are arranged at the center thereof, as shown in FIGS. 4A and 4B of Japanese Patent Laid-Open Publication No. 179391/1983 (corresponding to U.S. patent application Ser. No. 360,390 filed on Mar. 22, 1982). The above-described conventional BWR core is shown in FIG. 2 of the Japanese Patent Laid-Open Publication. The enrichment of each of the aforementioned fuel assemblies is decreased in an upper portion thereof in an axial direction thereof in order to increase the cold shutdown margin of a reactor, as shown in FIGS. 4C, 5A and 5B of the Publication.
It is also possible to increase the cold shutdown margin of a reactor by increasing a total cross-sectional area of the water rods in each of the fuel assemblies. An example of such fuel assemblies is shown in FIG. 2 of Japanese Patent Laid-Open Publication No. 178387/1984 (corresponding to West German Patent Application No. 3309449.7 filed on Mar. 16, 1983). This fuel assembly comprises fuel rods which are arranged in 9 rows and 9 columns (9.times.9) in a channel box and five water rods, each of which has a square cross-sectional form, arranged at the center of the channel box. The five water rods are arranged in a cruciate form.
A fuel assembly with a high level of burn-up has been now developed with the aim of achieving effective utilization of uranium resources. One of methods of obtaining a high burn-up type fuel assembly is to increase the average enrichment of fuel assembly. However, any increase in the average enrichment of the fuel assembly causes a decrease in the cold shutdown margin of the reactor concerned.